APPENDIX TO CHAPTER XV 229 



month by the process of division. Apparently the divi- 

 sion cannot go on indefinitely, but from time to time the 

 Paramecia unite temporarily in pairs and undergo an 

 exchange of some of their nuclear matter. This is doubt- 

 less the beginning of what is known in the higher animals 

 as sexual reproduction. 



APPENDIX TO CHAPTER XV 



KEY TO THE FOUR CLASSES OF PROTOZOA 



a\. Without non-retractile appendages. 



bi. With retractile pseudopodia .... Rhizopoda 



(Ex. Amoeba) 



6 2 . Without pseudopodia ; covered with an im- 



perforate cuticula ; parasites . . . Sporozoa 

 2 . With non-retractile appendages. 



61. No cilia, but with one or more flagella . . Flagellata 



(Ex. Euglena) 



6 2 . With cilia, or sucking tentacles . . . Infusoria 



(Ex. Paramecium) 



KEY TO THE SUBCLASSES AND ORDERS OF INFUSORIA 



a\. With vibratile cilia and no sucking tentacles . CILIATA 



b\. Body everywhere closely beset with cilia. 



GI. No adoral zone Holotricha 



(Ex. Parameciuiu) 



C 2 . Adoral zone present .... Heterotricha 



(Ex. Stentor) 



6 2 . Body only partly ciliated. 



GI. Cilia limited to ventral side . . . HypotriGha 

 c 2 . Cilia form a circlet around or at upper 



edge of animal Peritricha 



(Ex. Yorticella) 



a 2 . With sucking tentacles . . ... SUCTORIA 



